Rivers and streams carry invisible bits of soil and plant matter from the landscapes they drain. These small pieces of once living material, known as organic matter, are a source of food for bacteria living in the stream. As organic matter drifts past these hungry bacteria, they eagerly snatch it from the water, eat it, digest it, and burp out carbon dioxide. These belches of carbon dioxide bubble out of the stream and into the atmosphere, much like an open can of soda. So while streams can simply carry organic matter downstream (ultimately to the ocean), they can also act as reactors that covert it to carbon dioxide which then directly enters the atmosphere. This is important because carbon dioxide is a heat-trapping gas. Furthermore, this process can take place anywhere in the watershed – from the tiniest trickle to the widest river. In other words, it is easy to underestimate how much carbon dioxide is bubbling out of the watershed if you are only studying large rivers.

In the Arctic, the permafrost, or frozen soil, contains a lot of old soil and plant matter. It is sort of like a vast freezer for ancient organic matter. Thousands of years ago, this organic matter was perhaps part of a plant that lived while mammoths and wooly rhinoceros were stomping around nearby. Now, rising temperatures in the Arctic are opening the freezer door, causing more and more of the permafrost to thaw which in turn releases this ancient soil and plant matter into neighboring streams. It is unclear whether this thawed organic matter is the type of food that stream bacteria like to eat. We are studying the fate of this old permafrost organic matter when it ends up in a small arctic stream.

To test this, we gathered frozen permafrost soil and leached it like tea into a water sample taken from the stream. We then poured this strong permafrost tea back into the stream, and measured the amount of organic matter at specific distances downstream. We found that the amount of organic matter decreased downstream, which suggests that the hungry bacteria were removing it from the water. We also found that the amount of carbon dioxide increased in the streams after we added the permafrost tea. Together, these findings suggest that bacteria do like the taste of old permafrost organic matter and that streams are an important pathway for the conversion of this ancient organic matter to a heat-trapping gas in the modern atmosphere.